To reproduce:
-add a RadGroupBox and from the smart tag collapse its header
-add a RadGridView inside the RadGroupBox and dock the grid to fill the parent content. RadGridView does not occupy the whole RadGroupBox content.

Workaround:
public Form1()
{
    InitializeComponent();
 
    this.radGroupBox1.GroupBoxElement.Header.Visibility = Telerik.WinControls.ElementVisibility.Collapsed;
    this.radGridView1.Dock = DockStyle.None;
    this.radGridView1.Location = this.radGroupBox1.Location;
    this.radGridView1.Size = this.radGroupBox1.Size;
    this.radGroupBox1.Resize += radGroupBox1_Resize;  
}
 
private void radGroupBox1_Resize(object sender, EventArgs e)
{
    this.radGridView1.Size = this.radGroupBox1.Size;
}

Second scenario:
- add a RadGroupBox and a RadGridView inside it. Change the RadGridView.Dock property  to Fill.
- increase the GroupBoxElement.Header.Font's size or change the HeaderPosition . The child control does not fill the entire available space. There is empty space at the top of the RadGroupBox.